by Hydraulic Institute

Q. What is NPSH3, and what are the methods for determining the NPSH3 of a rotodynamic submersible pump?

A. NPSH3 is the net positive suction head available to a pump under test at a constant rate of flow when the pump head is decreased by 3 percent as a result of cavitation caused by a decreasing available suction head. Sometimes NPSH3 is referred to as net positive suction head required (NPSHR); however, a pump's NPSHR must be higher than the NPSH3 for the pump to operate without head reduction, and it may need to be higher than the NPSH3 for long-term reliable operation. For more information on the required margins above NPSH3, refer to ANSI/HI 9.6.1 Rotodynamic Pumps Guideline for NPSH Margin.The NPSH is established through the datum elevation of the impeller. The flow toward the pump must be uniform and free of undue disturbances. A pump tested with suction piping may require a flow-straightening device before entering the pump. Arrangements for cooling or heating the liquid in the loop may be needed to maintain the required temperature.

Multiple arrangements can be used to determine the NPSH3 characteristics of rotodynamic submersible pumps. One arrangement is shown in Figure The pump is supplied by a constant-level supply through a throttle valve followed by a section of pipe containing straightening vanes or a minimum of seven diameters of straight pipe to straighten flow. This arrangement decreases the turbulence produced by the throttle valve and makes possible a more accurate reading of suction pressure at the pump inlet.

Figure Suction throttling NPSH test setupFigure Suction throttling NPSH test setup

This simple arrangement usually is satisfactory for NPSH greater than 3 meters (10 feet); however, the turbulence at the throttle valve tends to accelerate the release of dissolved air or gas from the liquid as the pressure on the liquid is reduced. A test made with this arrangement usually indicates higher NPSHR than what would be expected with deaerated liquid.

Figure shows a second option. The pump is supplied by a sump in which the liquid level can be varied to establish the desired NPSHR. Be careful to prevent entrained air or vortexing as the liquid level is varied. The priming connection should be installed above the eye of the impeller, either in the discharge pipe or on the pump.

Figure Variable-lift NPSH test setupFigure Variable-lift NPSH test setup

For more information on test methods for rotodynamic submersible pumps, refer to ANSI/HI 11.6 Rotodynamic Submersible Pumps for Hydraulic Performance, Hydrostatic Pressure, Mechanical, and Electrical Acceptance Tests.

Q. How does axial thrust compare among impeller types for a rotodynamic vertical pump?

A. The net axial downthrust force is carried by the pump shaft. The shaft will stretch under this load. Before the pump starts, any stretch that occurs is the result of the sum of the static forces. The thrust load will increase after the pump starts because of the addition of dynamic forces.

The dynamic forces creating thrust on a vertical turbine pump enclosed impeller result from the difference in pressure distributions on the upper and lower shrouds along with the force from the change in momentum of the flow through the impeller (see Figure

Figures, b & c. The first (left) shows the enclosed impeller plain top shroud, the second (middle) displays the semi-open impeller and the third (right) illustrates the enclosed impeller with back ring and balance holes.Figures, b & c. The first (left) shows the enclosed impeller plain top shroud, the second (middle) displays the semi-open impeller and the third (right) illustrates the enclosed impeller with back ring and balance holes.

The semi-open impeller has only an upper shroud (see Figure The difference in pressure distributions along the shroud's backside and vaned side is typically greater than between the upper and lower shrouds of an enclosed impeller. Semi-open impeller axial thrust is higher than that of the enclosed impeller.

The axial flow pump impeller has no upper or lower shroud; vanes are attached directly to the hub. The axial thrust generated is primarily from dynamic forces created by interaction of the propeller vanes with liquid.